SUMMARY Deciphering how an axon grows, forms synaptic connections, and terminates axon outgrowth is essential if we are to understand how a nervous system is built. Such knowledge will be essential in harnessing the robust and resilient nature of the developing nervous system to design novel therapies for treating neurodevelopmental and neurodegenerative diseases. The long-term goal of our research program is to understand the cellular, developmental and molecular mechanisms that regulate synapse formation and axon termination. To this end, we rely upon C. elegans, a powerful model system that can be used to understand the fundamental principles governing construction of neural circuitry. The signaling protein RPM-1 is a key regulator of synapse formation and axon termination. Growing genetic evidence has linked RPM-1 signaling pathways to developmental disorders, such as schizophrenia and intellectual disability. The RPM-1 orthologs Highwire and Phr1 are also central regulators of axon degeneration. Thus, understanding how RPM-1 functions and how RPM-1 is regulated could significantly inform a range of conditions. In this proposal, we will further our understanding of how RPM-1 functions in axon termination and synapse formation on several levels. 1) We aim to understand on a cellular, developmental and molecular level why synapse formation defects occur in rpm-1 mutants. Our approach relies upon developmental time-course and real-time imaging to assess cellular deficits at the presynaptic terminals of rpm-1 mutants in order to test whether RPM-1 regulates synapse assembly or stability. We explore two mechanisms by which RPM-1 potentially affects synapse formation specifically, a putative MIG-15/NSY-1/JKK- 1/JNK-1 MAP kinase pathway and the tubulin acetyltransferase, ATAT-2. 2) We aim to determine whether RPM-1 functions in axon termination by regulating growth cone collapse. To do so, we rely upon in vivo cellular imaging, molecular genetics, and biochemistry. Our preliminary observations suggest that a Rac GTPase, MIG-2, is an important regulator of axon termination and growth cone collapse, which functions in the RPM-1 pathway. This is the first potential link between RPM-1 signaling and an actin regulator. 3) We aim to understand how RPM-1 is regulated. In preliminary studies using pharmacology and genetics to alter microtubule stability, we found evidence that RPM-1 regulates microtubule disassembly. This has revealed the intriguing possibility that a Tau-like protein, PTL-1, is a potential upstream, negative regulator of RPM-1, which we will test. In a complementary approach, we will explore the extracellular cues that regulate axon termination. In doing so, we aim to reveal further upstream regulators of RPM-1. Our observations suggesting that PTL-1 could regulate RPM-1 is particularly interesting given the strong links between the ortholog of PTL- 1, Tau, and neurodegenerative diseases, such as Alzheimer's disease.